1. Field of the Invention
The present invention relates to a vehicle suspension system and more particularly to a suspension system in which the toe direction of the wheel can be changed in accordance with a running condition of the vehicle.
2. Description of the Prior Art
Recent developments in motor vehicles includes an improvement in the suspension system so that the toe direction of a wheel, particularly a rear wheel is controlled in accordance with a running condition of the vehicle to thereby provide the vehicle with a desirable running property. For example, the U.S. Pat. No. 4,621,830 issued on Nov. 11, 1986 to S. Kanai discloses a vehicle suspension system in which the toe direction of a rear wheel is changed under a sidewardly directed force acting from the road to the wheel in a manner that the rate of change in the toe direction is increased as the side force is increased beyond a certain value. For example, the toe direction of the wheel is changed with a first ratio with respect to a change in the side force under the certain value of the side force, but the ratio is increased when the side force is increased beyond the certain value. With this structure, it becomes possible for example, as shown by the line b in FIG. 14 of the patent, to produce a toe-in movement in the rear wheel under a fast cornering operation or a lane change operation of the vehicle wherein the rear wheel is subjected to an increased side force to ensure a stable road gripping while suppressing a toe-in movement under an operation wherein the side force is small to permit a turning of a relatively small radius.
As an alternative solution, proposals are made to provide the vehicle suspension system with a hydraulic actuator for controlling the toe direction of the wheel. A detector is provided for detecting the side force in terms of a sideward acceleration so that the hydraulic actuator is operated when the side force is increased beyond a predetermined value to produce a toe-in movement of the wheel.
The conventional arrangements described above are designed so that the rear wheels are shifted in the toe-in direction as the side force is increased to thereby obtain an improved steering stability. It is also expected that the stability in a straight road operation can also be improved by this toe control. This is based on the concept that the vehicle running stability can be improved by shifting the rear wheels in a toe-in direction to thereby provide an improved road gripping of the wheels.
It should however be noted that the toe control of the rear wheels under the side force is not satisfactory in obtaining an improved stability in a straight road operation. This is particularly true under a high speed vehicle operation.